54 



studies in luminescence;. 



The methods of measurement were the same as those described in Chap- 

 ter III. The experimental data obtained are contained in Tables 13, 14, 15. 

 In each case it is the average of from ten to twenty observations that 

 is recorded. 



In the set of observations contained in Table 13 the curves for 0.547^, 

 0.483 ijl, and 0.512 fi were determined during the forenoon in the order just 

 stated. The data for 0.5 1 2 n with weaker excitation were observed during 

 the afternoon of the same day. The observations recorded in Table 14 

 were taken in a different manner, the measurements at a given intensity 

 being made for each of the three wave-lengths in succession. The obser- 

 vations of December 13 (Table 15) were made in a similar manner. The 

 first three curves of Table 13 are plotted in Fig. 44, while the second and 

 fourth curves of this table are shown in Fig. 45. 



Table 13. [Nov. 25.] 



Time in seconds required for the phosphorescence to fall from its initial intensity to 

 the intensity I. The computed values of / are derived by substitution in the 

 following equations: 



For X = 0.483^1 i/| 



X = o.5I2m i/I 



X = o.547/x i/I 



X = 0.512,1* (weaker excitation) i/| 



7 =0. 102 + 0.059^ 

 I =0.074 + 0.045/ 

 I =0.096 + 0.055/ 

 J =0.092 + 0.052/ 



Table 14. [Dec. 10.] 



Time in seconds required for the phosphorescence to fall to the intensity I. The 

 computed values of / are derived by substitution in the following equations: 



For X = o. 512/i 1/^/7=0.078+0.034/ X = o.547/j 1 /-y// =0. 104+0.042/ 



